The application relates to an acoustic system having a housing with adsorbent powder.
In acoustic systems having hollow spaces, such as, for example, loudspeaker housings, the compliance of the mostly air-filled hollow space, volume determines their effect at low frequencies, regardless of whether they are sound-generating or sound-attenuating systems. It holds that the greater the volume, the greater the acoustic compliance and the greater the acoustic efficiency. Manipulation is therefore required in order to increase the acoustic compliance of small hollow spaces when space is limited. Attempts at achieving this have led, among other things, to filling the housing with absorbent or adsorbent materials. Known porous materials have shown to be applicable here, since absorption is, simply put, a volume effect. The effect with reference to the magnification of the acoustic compliance is, however, limited in theory as well as in practice, above all because the volume represents precisely the critical magnitude. Adsorption, on the other hand, is a surface effect that theoretically can be extremely increased, as long as the effective surface can be increased regardless of volume. Effects were obtained with different adsorbers in different material form, for example, U.S. Pat. No. 4,657,108 and US 2004/0251077, which holds out the prospect of a doubling of the acoustic compliance. It is in practical terms for the most part active carbon, which is used as a coating for moldings or in granular form in the housing, as shown in FIG. 1, which will be described in more detail later. Active carbon is comparatively inexpensive and easy to obtain. However, it also has to disadvantages for this specific application: one of which is vulnerability to air humidity. The technical embodiments for housings filled with active carbon, in particular loudspeakers, are therefore concentrated on the protection from and the prevention of humidity in housings. Additives, such as substances with, in turn, absorbing or hydrophilic is properties, are proposed on the one hand to bind the humidity (US 2004/0251077). Additional barriers or packings in the housing are described, which on the other hand are to keep humidity away from the active carbon (U.S. Pat. No. 4,657,108). Since these barriers, for example, films or high-density fleeces, however also have an acoustic effect, partial compromises must be accepted. Pressure equalization tubes, which however can again contain active carbon to preclude the penetration of humidity, are provided in order to prevent, for example, a pressure difference between the housing and the active carbon packed in the film (U.S. Pat. No. 4,657,108).
An easy way to solve the problem of penetrating humidity is represented by a moisture-tight housing with a likewise tight loudspeaker and a tight connection between the housing and the loudspeaker. This configuration has remained unacknowledged so far for unknown reasons. A presumed reason is represented by the preferred use of electrodynamic conical loudspeakers. Waterproofing is not readily possible for the intended use. A problem is created at the same time if granular active carbon or powdered active carbon is used, since it can obstruct the open annular gap within which the oscillator coil moves. This results in another reason for the described protective measures with regard to active carbon in the housing.
Aside from these use-related configuration aspects, the question arises of whether the acoustic compliance can be further increased. A grain size of between 0.1 and 0.3 mm (U.S. Pat. No. 4,657,108) is preferred with reference to the surface effect adsorption and the previously known doubling through active carbon granulate. Active carbon in still finer disintegration (approx. 0.05 mm) is indeed proposed in the already mentioned pressure equalization tubes (U.S. Pat. No. 4,657,108), but with the express function of a moisture barrier. Further attempts or research with other modes of administration of active carbon are not known.
A loudspeaker arrangement is known from EP 1 868 410 A1. A housing is present therein. A housing wall is formed in part by a membrane. A drive, which stimulates the membrane to vibrate, is located inside the housing. There is furthermore an area within the housing, in which the powdered active carbon is arranged. The active carbon is accommodated in a sleeve, for example, a bag. In this way the active carbon is above all prevented from moving freely within the housing and damaging the drive that stimulates the membrane to vibrate. It should be noted in this connection that such drives are mostly coils with an annular gap. The drive can be damaged if the fine powdered active carbon reaches there.
It is generally known that active carbon can adsorb gas, depending on the pressure. It is occasionally erroneously addressed as absorption, for example, in the description in Wikipedia (status as of Jun. 4, 2013).
Kundt's tube is likewise described in Wikipedia. With Kundt's tube it is possible to make standing waves visible in a glass tube. Clubmoss spores, for example, are moved by the intensive sound wave and collect at the points at which the particle velocity of the sound waves is lowest, that is, in the nodes of the standing wave. The clubmoss spores are moved by the sound waves.
Starting from the previously determined potential of an active carbon filling in a housing for the purpose of increasing the acoustic compliance and the practical problems simultaneously connected therewith, the object is to reinforce the acoustic effect as well as to reduce or eliminate the practical problems.